The present invention concerns a method of producing cold-rolled, high-strength steel strip with good plasticity and isotropic properties as recited in the preamble to claim 1. Such steels and their composition are state of the art.
Cold-rolled steel strip is employed to manufacture a wide range of cold-formed products. Various forming procedures require steels with different properties.
The increasingly exacting demands of engineering and industry require better and better mechanical properties (characteristics), especially plasticity. Plasticity is defined by the highest possible r, n, and expansion, whereby r represents deep drawing, n stretching, and expansion planar strain.
It has been demonstrated practical for plasticity to be as equal as possible along the different directions, especially longitudinal, transverse, and diagonal, extensively isotropic in other words. When r is isotropic, .DELTA.r will be very low, and rotationally symmetrical pressings will be extensively free of ears. The advantages of isotropy are particularly expressed in uniform rheology and less waste.
Light-weight structural engineering is also an expanding field and demands thinner sheet metals. The sheet must be stronger to compensate.
To minimize the unavoidable decrease in plasticity that accompanies increased strength is accordingly a major goal of materials science.
A wide range of high-strength steels appropriate for cold forming is available at the state of the art. The present situation with respect to micro-alloyed and P-alloyed steels, bake-hardened or not, is essentially described in Stahl-Eisen-Werkstoffblatt 093 and 094. Bake-hardened properties can be particularly well brought out by one of the new continuous heat treatments, sometime in conjunction with hot dipping. Clean strip with uniform properties can easily be obtained.
Isotropy has long been easy to obtain. Rotationally symmetric pressings from an isotropic material will not have ears. One example is described in the Brockhaus B-Faktor advertisement in Der Spiegel, 19 (1966), 125. This example, however, does not expressly address the production of high-strength steel and requires either very special cold rolling or even standardizing annealing to eliminate ears.
A high-strength thin-sheet steel alloyed with titanium to eliminate ears has very recently become known from German Patent 3 803 064. The process is limited, however, to batchwise annealing and accordingly lacks the advantages of continuous annealing and of the finish provided by dipping. The potential for increasing such strength characteristics as yield point to approximately 220 to 280 N/mm.sup.2 is also limited. Another drawback is the strictly low r of 1.0, which is detrimental to deep drawing. Again, the high strength is essentially attained by the mechanism of compaction through reducing the grain size. A small grain means comparatively expensive temper rolling. Regular temper rolling entails the risk of flow lines and hence the failure of outer-skin areas. The relatively careful temper rolling necessary in the present event, however, is more detrimental to plasticity than ordinary temper rolling is.
Limitation to the almost exclusive effect of grain-size reduction by way of titanium also necessitates the precise matching of hot-rolling, cold-rolling, and annealing conditions to the particular chemistry, accompanied by high demands for precision. Another drawback is the restriction of final rolling temperatures to above A.sub.r3, whereby the rolling of strip with low final thickness is in particular more difficult because of the associated higher temperature loss.